1. Field of the Invention
The present invention relates to a method for treating exhaust gases containing nitrogen oxides, such as exhaust gases resulting from the combustion of various petroleum fuels and fossil fuels, as well as waste gases from nitric acid manufacturing plants and pickling plants, for the effective removal of the nitrogen oxides. More particularly, it relates to a chemical treatment of such exhaust gases wherein exhaust gas is treated with neutral or acidic aqueous solutions of urea in the presence of a selected catalyst.
2. Description of the Prior Art
Exhaust gases resulting from the combustion of petroleum fuels such as crude oil, heavy oil, light oil, kerosene, light distillate, LPG and LNG, and of fossil fuels such as coal, usually contain about 10 to 1,000 ppm of nitrogen oxides. Waste gases from nitric acid manufacturing plants and pickling plants normally contain several thousands or more ppm of nitrogen oxides. When these amounts of nitrogen oxides are released into the atmosphere, they can cause serious pollution problems.
Various methods have heretofore been proposed to remove the nitrogen oxides from the exhaust or waste gases containing the same. These methods may roughly be classified as "dry" and "wet" methods.
Known dry methods include, for example, selective reduction wherein the nitrogen oxides are selectively reduced in the presence of a suitable catalyst with a reducing agent such as ammonia; non-selective reduction wherein the nitrogen oxides are reduced with a reducing agent such as hydrogen, carbon monoxide, methane or the like; adsorption wherein the nitrogen oxides are adsorbed with a suitable adsorbent such as active carbon, and catalytic decomposition wherein the nitrogen oxides are decomposed through the use of a suitable catalyst.
The known method, however, based on the selective reduction of nitrogen oxides involves disadvantages such as poisoning of the catalyst by sulfur oxides, plugging of the catalyst bed by dust contained in the combustion exhaust gas, the emitting of a considerable amount of the reducing agent with the treated exhaust gas and a low percentage of removal of the nitrogen oxides. The method, therefore, has not yet been practiced commercially. The method based on non-selective reduction has similar disadvantages. Additionally, the reducing agent may be consumed by oxygen contained in the exhaust gas. Accordingly, this method is also impractical. The known method based on adsorption involves such problems that a satisfactory adsorption capacity cannot be obtained with any of the known adsorbents and that components other than nitrogen oxides in the exhaust gas may often be adsorbed preferentially. With respect to the method based on the catalytic decomposition of nitrogen oxides, no satisfactory catalysts have been found.
As to the wet methods, various methods have been proposd in which exhaust or waste gas is contacted with an aqueous solution containing various adsorbents, such as alkalis and acids. Because of a low reactivity of the nitrogen oxides, however, the attainable absorption efficiency is generally low. Regeneration of the spent absorbent is required and makes the process complicated. Furthermore, a problem with respect to the treatment or disposal of the by-products is involved. For these reasons and others, none of the known wet processes has been commercially practiced for the treatment of large amounts of combustion exhaust gases.
Accordingly, it is a primary object of the present invention to provide a method for the treatment of exhaust or waste gases containing nitrogen oxides and originating from various sources, which method is extremely simple, effective and economical for the removal of the nitrogen oxides.